The present invention generally relates to the field of hydrodynamic bearings and, more specifically, to an improved compliant foil thrust bearing set providing for a reduced startup torque requirement.
Hydrodynamic bearings are utilized in many types of powered mechanical systems. These bearings are generally placed between two relatively-movable structural components, conventionally referred to as a rotating thrust runner and a stationary housing, where a working fluid is used to prevent dynamic contact between the structural components. The working fluid found in hydrodynamic bearings can be either a liquid such as oil, water, or refrigerants, or a gas such as nitrogen, oxygen, methane, hydrogen, refrigerants, or air. As low viscosity working fluids can be provided for bearing lubrication, hydrodynamic bearings are particularly suitable for applications in which the runner moves relative at a high rate of rotation relative to the housing.
The hydrodynamic bearing may also include one or more foil components in the region between the thrust runner and the bearing housing. The foil components are usually fabricated from a thin sheet of compliant material such as beryllium copper, nickel alloys or stainless steel. Use of the foil material enhances the hydrodynamic characteristics of the hydrodynamic bearing in that deflection of the compliant foil produces desirable hydrodynamic pressure forces between the structural components. It has been demonstrated that such compliant foil components serve to improve bearing function, even under adverse operating conditions.
Unfortunately, the past methods and apparatus for providing hydrodynamic bearing support have disadvantages. To maintain the desired deflection of the compliant foil, the compliant foil hydrodynamic fluid film thrust bearing may include a backing spring to pre-load the compliant foil against one of the structural components. This is exemplified by U.S. Pat. Nos. 5,498,082 and 6,354,741 in which a backing spring is used to hold a foil component in contact against a rotating thrust runner.
In such pre-load configurations, the backing spring provides an axial force which serves to control the deflection of the compliant foil, and helps to establish the converging wedge required to generate the fluid pressure forces, that support the desired thrust load. However, this pre-load force undesirably serves to increase the starting torque of the bearing, where the starting torque should ideally be kept at a low value. Moreover, as the axial pre-load force acts to retain the compliant foil in contact with the thrust disk, there also results an increase in the required thrust disk speed at which the hydrodynamic effects in the wedge channels are strong enough to overcome the pre-load force and force the thrust disk out of physical contact with the top foil. Consequently, such use of a pre-load force results in an increased lift-off/touch-down speed and may increase measurable bearing wear each time the rotating component is started or stopped.
In another configuration, exemplified by U.S. Pat. No. 5,529,398, a spacer is incorporated between two axially-adjacent thrust bearings to provide a space slightly wider than a two-sided thrust disk positioned between the thrust bearings. The spacer is clamped between the adjacent bearings. It can be appreciated by one skilled in the relevant art that if thermal gradients are present in the thrust bearing, this clamping force will result in bearing distortion and bearing performance will be reduced.
As can be seen, there is a need for an improved apparatus and method that provides a hydrodynamic foil bearing with a reduced startup torque requirement.